In the past, waterbed heaters have used various thermal sensing means in order to prevent the overheating of the waterbed heater with its consequent damage to items in contact with the heater, such as a waterbed mattress. Thus, Trostler, U.S. Pat. No. 3,982,098, discloses a temperature-sensitive fuse, item 23 in FIG. 2, encapsulated between two pieces of plastic. Trostler also discloses over-temperature control circuitry utilizing thermistors, T-1 and T-2 in FIG. 6, encapsulated between two pieces of plastic. Miller, U.S. Pat. No. 3,840,985, discloses an overheat thermostat, item 38 in FIG. 5, built into the heater.
While these devices do provide some protection against overheating, the overheating protection devices used in the prior art do not guide against the possibility that a waterbed mattress will be partially drained, leaving a portion of the waterbed heater covered only by the waterbed mattress, liner, bedding and similar materials, and not covered by water. In such a situation, the portion of the waterbed heater that is not covered by water may overheat.
In addition to sensing an overheat condition, prior art waterbed heaters may additionally use a variety of means to sense the temperature of the water in the waterbed. Hall, U.S. Pat. No. 3,585,356, discloses a temperature sending bulb, item 66 in FIG. 8. McMullan, U.S. Pat. No. 4,352,976, discloses a temperature-sensing device, item T in FIG. 9, molded into the heater cord. As used in the prior art, temperature control sensing means differ from overheat sensing means in that overheat sensing means as employed by the prior art react only to an overheat condition, while the temperature control sensing means of the prior art react only to a narrow range of temperatures which does not include the temperatures found during an overheat condition. By the present invention it is desired to build a system in which a temperature control sensing means can be used to detect an overheat condition, yet still respond with adequate sensitivity to the temperature changes found during normal operation of the waterbed heater. Such temperature control sensing means can be used alone, or in conjunction with prior art overheat sensing means.
It is desired to provide a system for detecting and/or preventing the localized overheating of a waterbed heater, as described above. It is further desired to build a heater that will operate for long periods of time under abnormal conditions, such as partial or full drainage of the waterbed, without damage to the waterbed system, which system includes the heater, waterbed mattress, waterbed liner, bedding, and other materials. It is also desired to provide a means by which water temperature may be controlled to within a very small range of temperatures, for exmaple, 1.degree. Fahrenheit, when the waterbed heater is operating under normal conditions, and to continue to control the waterbed heater surface temperature to a safe level when water is partially or wholly removed from the waterbed mattress.